You must use a break-in additive meant for use with flat tappet cams. A good parts store will have it.
You want the engine to start with as little cranking as possible. Take the covers off the SU float bowls and fill them halfway with gas (then put the covers back on, of course). Fill the oil filter halfway and then spin it on quickly. Make sure the ignition timing is close to right. The battery should be fully charged. Don't forget to hook up the engine ground strap.
Start the engine and run it at 2000 RPM for at least 20 minutes. You can rev it higher than that briefly, but do NOT let it idle. Then turn it off, drain the oil and toss out the filter. Let it all cool down fully.
Replace the oil and filter (if you have some break-in additive left, put that in, but it's not strictly necessary). Remove the rocker assembly and retorque the head. Put the rockers back on and readjust the valves. Start the engine and warm it up at a fast idle, then you can let it idle long enough to tune the SUs and check the timing. As soon as possible, drive the car -- you don't need to go find the redline just yet, but use plenty of throttle. You need pressure in the cylinders to seat the rings. If you don't do this early, they may never seat fully.
Work progressively up to higher revs over the first 150 km or so. After that, you can drive it however you want.
When you put the modified head on, you don't need to break anything in over again. You do need to do the retorque and readjust the valves.
